Guy > Kisame
Itachi = Kakashi
there you have it
Fixed that for you.
Gai is the ultimate counter to Kisame. His gates repel water, his speed isn't something Kisame can match, he can produce enough power to negate Kisame's regen abilities, etc. Only reason he didn't kill him with Hirudora(or when he punched him after he twitched) was so he could be interrogated.
Assuming the knowledge is manga(or full, whichever), Kisame would probably defer Gai to Itachi and go after Kakashi. His mass amount of chakra reserves+samehada's ability to absorb chakra counters Kakashi's average at best chakra reserves. This would also take Itachi away from one of the only people in the manga who can match him as a strategist and one of the only people who possesses a technique that can instantly spell doom for Itachi.
Gai's hirudora has shown to be able to do damage to susanoo, but Itachi has the Yata Mirror to negate that threat. He could pretty much wait for Gai to tire out from Gates use, or the quicker way of dealing with Gai is using amaterasu to defend, somewhat similar to what Sasuke did to Raikage, only Itachi would be smart enough to know even if Gai gets touched by the Amaterasu, it would only take one hit from Gai to obliterate Itachi in gates. Gai might lose an arm, but Itachi would lose his life. Therefor, Susanoo+amaterasu is his best bet, as I doubt the totsuka blade would be fast enough.
Kisame's water dome isn't a factor here because of him being close to Itachi, and not wanting to trap him in it, but he'll be able to hold off Kakashi long enough for Itachi to deal with Gai, assuming Kakashi doesn't resort to kamui.
If it goes Gai vs Kisame and Kakashi vs Itachi, Gai stomps Kisame and helps Kakashi deal with Itachi...just depends on which way you feel the strategies would work out.